The present invention pertains to a pigment based inkjet ink and ink set. More particularly the invention pertains to an ink that is red, green or blue in color comprised of certain pigment combinations. The invention further pertains to an ink set comprised of at least one red, green or blue ink, and most preferably to an ink set with at least six differently colored inks that includes a cyan, magenta, yellow, red, blue and green ink.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on a substrate, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor. Inkjet printers offer low cost, high quality printing and have become a popular alternative to other types of printers.
Color characteristics of pigment-containing inks play an important role in the quality of the printed ink-jet image. Perceived color quality can be characterized using any one of several color-space systems such as CIELAB (CIE No. 15.2, Colorimetry, 2nd. Ed., Commission Internationale de I'Eclarage, Vienna, Austria, 1986) or Munsell (Munsell Book of Color, Munsell Color Co., Baltimore, Md., 1929), as is well known in the art. With respect to Munsell color space, a given color is defined using three terms, namely Hue, Value and Chroma. With respect to CIELAB color space, a color is defined using three terms L*, a* and b*. In this system L* defines the lightness of the color and it ranges from 0 (black) to 100 (white). The terms a* and b* together define the hue, where a* ranges from a negative number (green) to a positive number (red) and b* ranges from a negative number (blue) to a positive number (yellow). Additional terms such as hue angle (ho) and chroma (C*) can also be used to describe a given color instead of a* and b* wherein
                              h          o                =                              tan                          -              1                                ⁡                      (                                          b                *                                            a                *                                      )                                              Equation        ⁢                                  ⁢        1                                          C          *                =                                            a                              *                2                                      +                          b                              *                2                                                                        Equation        ⁢                                  ⁢        2            
To achieve full color images, ink jet printers typically employ a cyan (“C”), magenta (“M”) and yellow (“Y”) ink. These colors are known as subtractively-mixing primaries, as light is subtracted as it passes through the colorant. When these colors are mixed in pairs they form red, green and blue (so-called “secondaries”), and when all are mixed together they form black. Thus these three color inks can be used to print the entire range of hues with good chromaticity and/or color saturation.
The range of colors that a given ink set can print is called the color gamut, and can be described in the CIELAB system by the volume in the L*a*b* color space that encompasses a palette of representative colors that can be printed by the ink set on a given substrate. While a CMY ink set can produce all possible color hues, the color gamut does not encompass all the colors that can be perceived by the human eye. Thus there is still a desire to improve the printed image quality and extend the range of colors that can be printed.
One method for improving image quality or color gamut is to expand the ink set used for printing to include additional inks. There are various ways to employ expanded ink sets.
One way is by including inks of the same hue but lower concentration (so-called “lights”). Light colors improve image quality by producing smoother tonal gradations and light colors in an image. For light colors, the “light” or “photo” inks, which typically have 10%-20% of the colorant content of the full-strength colors, are used. For intermediate strength colors, “light” and full-strength inks are mixed. For the darkest colors only full-strength ink drops are used. Thus a smooth gradation of color tones can be obtained.
A further advantage of using “lights” is that it allows the dye concentration of the full-strength inks to be increased further without having the problem of dark sparsely spaced dots on the image. The higher concentration inks produce higher chroma secondary colors when mixed on high quality photo papers, and they also produce a darker CMY process black. These two effects serve to enhance the color gamut significantly.
U.S. Pat. No. 5,833,743 discloses how up to three different concentrations of each of the primary hues can be used to greatly increase the color gamut obtainable from a CMY ink set. Typically “light” inks are only used for cyan and magenta as yellow is light enough so that any graininess is not objectionable.
U.S. Pat. No. 6,509,916 discloses a printing method where a light ink of a different hue is added to the three-color set in order to improve improved gradation reproduction and expand the color gamut.
Another way an expanded ink set can be employed is by including more inks with hues other than CMY. In some cases this additional ink may be what is known as a spot color, where the image to be printed contains an abundance of a certain color and so that color is specifically added to the ink set. This spot color is not necessarily outside the existing color gamut of the CMY ink set. In the case where the desired color is inside the gamut, adding the ink still provides the advantage of using less ink since a single ink is used rather than a mixture, and it also may provide improved light color tones, again by avoiding using a mixture of other colors. As another example the digital image to be printed may contain a certain color that is difficult to accurately print with the existing ink set and that color can be selected as an additional ink for the ink set.
All of the above-identified publications are incorporated by reference herein for all purposes as if fully set forth.
The art described above attempts to improve image quality by making piecemeal additions to the basic CMY ink set to address specific issues; however, none of the solutions in themselves provides both a range of colors able to reproduce photographs and computer monitor images as well as reproduce light color tones. In addition many of the solutions assume that it is a simple matter to add colors to the ink set. This may be the case in many conventional (non-inkjet) printing processes; however, in inkjet printing the number of ink slots available is both fixed by the printer design and also limited due to the scanning nature of much ink-jet printing, which limits the width and weight of the printhead.
Commonly owned U.S. application Ser. No. 11/105,258 (filed Apr. 13, 2005), claiming priority from U.S. Provisional Appln. No. 60/564,062 (filed Apr. 21, 2004), entitled “Inkjet Ink Set for Improved Color Reproduction” (the disclosures of which are incorporated by reference for all purposes as if fully set forth), describes a six color dye-based ink set.
There is, consequently, a need for an ink set that integrates (a) a systematic method of expanding the color gamut, with (b) a method of printing light tones, highlights and grayscales using (c) a limited number of ink channels. There is furthermore a need to achieve these image quality improvements with pigment-based ink to gain the inherently better color fastness of pigment colorants. It is an object of this invention to provide an inkjet ink and ink set that address these issues.